Sn-Pb alloys have been used in soldering since ancient times, and they are still most popular for soldering electronic devices to printed circuit boards or other substrates.
When they are to be discarded, electronic appliances including televisions, radios, audio or video recorders, computers, copying or printing machines, etc. may be disposed of in landfills, since they are composed of various materials such as synthetic resins used for housings and printed circuit boards, and metals used for wires and other electric connections, and frames, and are not suitable for disposal by incineration.
In recent years, the phenomenon of acid rain has become serious since the acidity of rain is increasing mainly due to discharge of sulfur oxide into the atmosphere by extensive use of fossil fuels such as gasolines and fuel (heavy) oils. Acid rain causes the solders used in discarded electronic appliances present in landfills to dissolve and penetrate into the ground. If such contaminated groundwater is ingested by a person for many years, the accumulation of lead in the person's body may result in lead poisoning (plumbism). From this viewpoint, there is a need of a lead-free solder alloy in the electronics industry.
Conventional lead-free solder alloys are Sn-based alloys such as Sn-Ag and Sn-Sb alloys. In Sn-Ag alloys, an Sn-3.5Ag alloy is the eutectic composition with a melting temperature of 221.degree. C. Even if this composition having the lowest melting temperature among Sn-Ag alloys is used as a solder alloy, the soldering temperature will be as high as from 260.degree. C. to 280.degree. C., which may cause thermal damage to heat-sensitive electronic devices during soldering, thereby deteriorating their functions or rupturing the devices. Of Sn-Sb alloys, an Sn-5Sb alloy has the lowest melting temperature, but its melting temperature is as high as 235.degree. C. in the solidus line and 240.degree. C. in the liquidus line. Therefore, the soldering temperature is in the range of from 280.degree. C. to 300.degree. C., which is still higher than that of the Sn-3.5Ag alloy, and thermal damage to heat-sensitive electronic devices cannot be avoided.
In view of the relatively high melting temperatures of Sn-Ag and Sn-Sb alloys as solder alloys, many attempts to lower their melting temperatures have been proposed. See, for example, Japanese Patent Applications Laid-Open (JP Al) Nos. 6-15476(1994), 6-344180(1994), 7-1178(1995), 7-40079(1995), and 7-51883(1995).
The solder alloys disclosed in these Japanese patent applications contain Bi and/or In (indium) in a large proportion in order to lower the melting temperatures. Although Bi and In are both effective for decreasing the melting temperatures of Sn-Ag and Sn-Sb solder alloys, the addition of Bi and/or In in a large amount is accompanied by the following problems. Addition of Bi in a large proportion makes the solder alloys very hard and brittle. As a result, it is impossible or difficult to subject the solder alloys to plastic working into wire, or when they are used to solder electronic devices, the soldered joints may be readily detached when subjected to only a slight impact. Addition of indium in a large proportion to solder alloys is undesirable due to its very high cost.
In order to avoid thermal damage to electronic devices during soldering, the soldering temperature should be 250.degree. C. or lower, and in order to perform soldering at such a temperature, it is desirable that the liquidus temperature of the solder alloy be 210.degree. C. or lower and preferably 200.degree. C. or lower.
However, with the above-described approaches to lower the melting temperatures of Sn-Ag and Sn-Sb solder alloys by addition of Bi and/or In, it is difficult to decrease the liquidus temperature of the alloys to 200.degree. C. or lower unless Bi and/or In is added in a large amount. Furthermore, even though it is possible to provide a solder alloy having a liquidus temperature lowered to 200.degree. C. or lower by such a means, the solidus temperature thereof, at which solidification of the alloy is completed, may also be lowered even more, so that it takes a prolonged period of time to completely solidify the solder alloy in soldered joints formed by soldering. As a result, if the soldered joints is subjected to any vibration or impact before they are completely solidified, they may be cracked.
Another problem of conventional lead-free solder alloys is that those lead-free alloys having liquidus temperatures which are low enough to be close to their solidus temperatures do not have satisfactory mechanical properties such as tensile strength and elongation, thereby forming soldered joints which have poor bonding strength or which are liable to be detached upon impact.
It is an object of the present invention to provide lead-free solder alloys having a liquidus temperature which is below 210.degree. C. and preferably below 200.degree. C. and a solidus temperature or peak temperature, at which solidification of the alloy is completed or substantially completed, is relatively close to the liquidus temperature.
It is another object of the present invention to provide lead-free solder alloys which have good bonding strength when used for soldering.
A more specific object of the present invention is to provide a lead-free solder alloy having the following properties.
1) It can be used at a soldering temperature below 250.degree. C. and preferably from 230.degree. C. to 240.degree. C. so as to prevent thermal damage to heat-sensitive electronic devices during soldering.
2) It has quite good solderability.
3) It has a narrow (solidification) temperature range between the liquidus and solidus temperatures (or peak temperate at which solidification is substantially completed) such that the alloy is rapidly solidified after soldering in order to prevent the resulting soldered joints from being cracked when vibration or an impact is applied immediately after soldering, the temperature range being close to the eutectic temperature of Sn-Pb alloy.
4) It produces soldered joints having a bonding strength which is high enough to prevent the joints from being detached upon application of an impact.
5) It can be easily subjected to plastic working into wire such that it can be used for soldering with a solder iron.